1. Field of the Invention
The present invention relates generally to manufacturing optical fibers, and particularly to a process for drying optical fiber coatings.
2. Technical Background
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business environments for internal data, voice, and video communications has begun and is expected to increase.
Optical fibers typically contain a glass core, a cladding, and at least two coatings, e.g., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequently attenuate signal transmission. This cushioning effect must be maintained throughout the fiber""s lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) which is lower than the foreseeable lowest temperature exposed to the fiber during use. This enables the primary coating to remain elastic throughout the useful life of the fiber and prevent microbending.
It is important for the primary coating to have a refractive index which is different (i.e., higher) than the refractive index of the cladding. This difference in refractive index creates a refractive index differential between the cladding and the primary coating that allows errant light signals to be refracted away from the glass core.
Also, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. Moisture resistance is essential for adequate adhesion and strippability because moisture affects the adhesion of the primary coating to the glass. Poor adhesion can result in microbending and/or various sized delaminations, which can be significant sources of attenuation in the optical fiber. To provide adequate adhesion during thermal and hydrolytic aging many primary coating compositions include adhesion promoters which facilitate adhesion of the primary coating to the glass fiber. Maintaining good adhesion of the primary coating to the glass cladding of the fiber is desired in order to ensure good attenuation performance.
A number of suitable adhesion promoters have been described in the art, including acid-functional materials and organofunctional halo-silanes or alkoxy-silanes. Of these, organofunctional silanes are preferred, because such silanes are less corrosive and coatings incorporating such silanes tend to better maintain their adhesive properties. Organofunctional silanes particularly provide adequate adhesion during thermal and hydrolytic aging. Suitable organofunctional silanes which have been described in the art include, generally, amino-functional silanes, mercapto-functional silanes, methacrylate-functional silanes, acrylamido-functional silanes, allyl-functional silanes, vinyl-functional silanes, and acrylate-functional silanes. Organofunctional silanes react with moisture/water within the coating and become activated to react with a siliceous surface or other activated silanes.
Coating raw materials can contain significant amounts of moisture/water and can lead to formulated liquid coatings with high water content. The water in the coating may react prematurely with the organofunctional silanes and thus allowing for self condensation of the silanes, which in turn can lead to a reduced shelf life of the coating and poor adhesion between at least one of the coatings and the optical fiber.
A need remains to control the reaction of the organofunctional silanes such that the silanes react with the glass cladding to create an adhesion between the cladding and the primary coating. The present invention is directed to overcoming at least this deficiency in the art.
The present invention relates to a method of drying an optical fiber coating. The method includes blending a plurality of raw materials capable of forming an optical fiber coating and dehydrating the plurality. One embodiment of the method includes drying the optical fiber coating by passing a dehydrating medium over a top surface of a body of the plurality of materials. A second embodiment of the invention includes bubbling a dehydrating medium through the body of the plurality of raw materials. In a third embodiment of the invention, the plurality is dehydrated by reacting the water in the plurality with a dehydrating medium that includes a compound having a functional group that is capable of reacting with the water.
One advantage of practicing the invention is that the adhesion between the coating and the optical fiber is increased. Increasing the adhesion between the coating and the fiber will reduce microdelamination of the coating and decrease any attenuation exhibited by the fiber associated with microbending. One excellent environment in which the reduction in microdelamination will be exhibited is a wet environment. One example of a wet environment is fiber exposed to water or high humidity conditions.
Another advantage which will result from practicing the invention is that the amount of moisture in the coating will be reduced to an amount less than the amount required to cause permanent degradation of the coating. An additional advantage that will result from practicing the invention is that the optical fiber coatings can be manufactured with a consistent moisture level. Also a coating dried in accordance with the invention will exhibit a longer shelf-life than an undried coating.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.